The first attempt to introduce sulfur into an alkaline earth metal salt of an alkylhydroxybenzoic acid was made by Orland M. Reiff, as disclosed in U.S. Pat. No. 2,256,443. He first synthesized an alkali metal salt of an alkylsalicylic acid by the Kolbe-Schmitt process and introduced sulfur into said salt by reacting it with sulfur chloride in the presence of a butyl alcohol solvent and thereafter obtained a corresponding alkaline earth metal salt using an alcoholate of an alkaline earth metal. This method was unique in that it successfully reduced the generation of hydrogen chloride during the reaction.
At a later time, Jerome M. Cohen also introduced sulfur into an alkali metal salt of an alkylsalicylic acid that was synthesized by the Kolbe-Schmitt process as described in U.S. Pat. No. 3,595,791. His method consisted of subjecting said alkali metal salt to double decomposition with an alkaline earth metal halide to form a corresponding alkaline earth metal salt and reacting it with elemental sulfur in the presence of a glycol monoether and an alkaline earth metal oxide or hydroxide. The major feature of this method is that elemental sulfur, which is easy to handle, is used in place of a highly reactive sulfurizing reagent such as sulfur chloride.
However, the two methods described above have experienced the following problems, which make them unsuitable for use in commercial operations. First, they are complicated processes involving many steps. In the Reiff process, the product obtained by a sulfurization reaction has to be converted to a free acid form before the final product is attained. In the Cohen process, double decomposition with an alkaline earth metal halide is necessary after the Kolbe-Schmitt reaction. Secondly, both processes involve a step in which an alkali metal halide is formed as a by-product. Inclusion of such a strong electrolyte into a final product is not suitable in view of its quality. Reactions involving the combination of alkaline earth metal complexes of alkylphenols with carbon dioxide have been utilized for many years in the phenate industry which has competed with the salicylate industry [see Nishikawa and Ishibe, PETROTECH, 7, 338 (1984)].
However, as already described in U.S. Pat. No. 2,916,454, such combinations are unable to form a detectable amount of salicylic acids. In this process, alkylphenol sulfides were reacted with metal alcoholates and the resulting complexes were subjected to treatment with carbon dioxide at 125-200.degree. C. and it was found that a carboxylation reaction did occur when the metal was an alkali metal, but not in the case where an alkaline earth metal was used.
One of the inventors of the present invention conducted studies on the process of phenate production comprising the steps of mixing and reacting an alkaline earth metal oxide or hydroxide, an alkylphenol, sulfur and a dihydric alcohol, thereafter distilling off water and excess dihydric alcohol, and subjecting the resulting sulfurized alkaline earth metal phenate to a treatment with carbon dioxide. In the course of these studies, he discovered the partial occurrence of a carboxylation reaction as disclosed in U.S. Pat. No. 4,123,371. What is interesting about this reaction is that it was carried out in the presence of a free alkylphenol although the presence of phenols had previously been considered to be deleterious to the formation of alkylsalicylic acids (see, for example, British Pat. No. 734,622, page 1, line 34 et seq).
The discovery of the above-described fact motivated the present inventors to develop sulfides of alkaline earth metal salts of alkylhydroxybenzoic acids. A thorough review of the factors associated with the conditions for this reaction (the study of which had led to the discovery described in U.S. Pat. No. 4,123,371 noted above), showed that problems still remained to be solved, not only in terms of the efficiency of carboxylation, but also in terms of the properties of the final product, including the oil solubility, stability and viscosity. However, the present inventors unexpectedly found that such aspects could be appreciably improved and a characteristic product could be attained by performing a sulfurization step after the treatment with carbon dioxide, rather than prior to it as had been the practice known in the art. The present invention has been accomplished on the basis of this finding.